INTRODUCTION. 2] 



through the Appalachian chain, the coarse sandstones and conglomerates 

 indicate the close of this period ; while the same geognostic line, from 

 the northern side of Lake Huron, by the course of the Cincinnati axis, 

 quite to the centre of Tennessee and still farther to the south, is marked 

 by bands of coral limestone. 



In the region of Eastern New- York, the coarse materials of mechanical 

 origin are accompanied by littoral or shallow-sea shells ; while farther 

 from the shore line and the influences of the stronger currents, the same 

 deposit became the habitation of other forms adapted to the changed 

 condition, and finally coral reefs occupied the bed of the ocean in the 

 vicinity of the present Ohio valley. 



The ocean bed of this geological period, like that of all others and of 

 the present epoch, was not uniform in its conditions, nor in the depth of 

 sea. There were certain lines of no great variation in depth, along which 

 accumulated the forms fitted for the conditions. The most prolific zone in 

 the limestone formations is that marked by the most perfect development 

 of corals ; and it is along this line, also, that other forms accumulate in 

 the greatest numbers. Where, on either side of this zone, the conditions 

 change, whether it be from deeper water, the deposition of arenaceous 

 and argillaceous material, or from whatever cause that corals and bryozoa 

 become less numerous, we soon find, likewise, a diminution in the other 

 forms of life ; until, as we approach the ancient sea margins, or explore 

 the muddy and sandy bottoms where no limestones occur, we sometimes 

 find an abundant fauna adapted to these conditions, and of the kind where 

 brachipods are in small numbers, and where corals are scarcely seen. 



We have been accustomed to look to the northeast for the source of the 

 sedimentary materials of this group; and to regard some part of the 

 present Northern Atlantic ocean bed as having been occupied by land, 

 the destruction of which furnished the sedimentary materials for this 

 formation. We are scarcely prepared, therefore, for the information which 

 comes as the result of investigations in the Canada Survey, that while 

 this source may have been to the northeast from us in New- York, and far 

 beyond the limits of our explorations, it lies in a direction more to the 

 east than we have been accustomed to believe. 



